For many years valve assemblies have been manufactured wherein the valve seat and its cooperating valve element are movable relative to one another by sliding contact to open and close the valve. One of the members of such a valve has been provided with a resilient and deformable valve facing for providing the necessary seal between the members. The facing is conventionally leather, but rubber has also been used. The valve facing must be deformable since it is necessary for effective sealing that the valve facing be very tightly wedged between the valve seat and valve element and to this end the valve facing in the uncompressed configuration is slightly larger than the available space between the valve seat and the valve element. However, the valve facing must also be resilient since it must again expand to the larger uncompressed configuration when the valve is in the opened position so that it may again be tightly wedged into the space between the valve seat and valve element when the valve is in the closed position. Also, since this wedging action places a severe abrasion or shear force on the valve facing, the material of the valve facing must be abrasion resistant in order to provide a minimum acceptable number of cycles of valve opening and closing. Both leather and rubber have acceptable properties in these regards, but on the other hand, the number of cycles of the valve with these materials is much less than would be desired.
A typical type of valve having a valve facing of the present nature is that of the drain valve of a "dry" barrel type of fire hydrant. These drain valves usually consisted of a drain passage from the exterior of the hydrant through the barrel or shoe and through a portion of the main valve seat assembly, the passage opening into the interior of the barrel at a point above the main hydrant valve when the same is closed. A valve facing strip is carried by the movable main valve element of the hydrant. This strip is arranged to have sliding contact with the valve seat assembly so as to close the opening of the drain passage to the interior of the barrel when the main hydrant valve element is moved to the open position.
Conventional leather drain valve facing strips can function effectively in such service, but they do have the serious disadvantage of deteriorating in time and allowing substantial leaking after a relatively low number of cycles of operation of the valve. Thus, they fail due to lack of continued resiliency and lack of abrasion and aging resistance.
More recently, efforts have been made to utilize drain valve facing strips made of rubber. These strips may be of a solid configuration, i.e. having a uniform cross-section somewhat similar to that of conventional leather facing strips, or they may be of a special configuration so that water pressure on one side thereof provides a seal. Both configurations, however, have not proven to be totally satisfactory when used over long periods of time as they have a tendancy to "cold-flow" and the required overall resiliency is lost and leakage occurs. Further, the rubber tends to deteriorate with age and further lose resiliency and abrasion resistance.
More recently, efforts have been made to utilize valve facing strips made of polyethylene, since this material is substantially more resistant to aging than rubber. These strips may be of a solid configuration similar to the leather strips but the very low order of resiliency of the material results in significant leakage with increased numbers of cycles of operation. In an effort to mitigate this problem, the polyethylene strips have been provided with a recessed configuration on one side thereof in order to increase resiliency. While both of the polyethylene strips provide greater numbers of cycles in drain valves than strips made with leather or rubber, they still have undesired increasing leakage with the number of cycles of operation.